1. Field of the Invention
The present invention relates to a low loss surface acoustic wave (SAW) filter and, more particularly, to a low loss SAW filter which utilizes apodized, group-type recessed electrodes to provide low insertion loss, high sideband rejection, and small passband ripple.
2. Description of the Prior Art
Surface acoustic wave filters often are of limited use due to ripple problems within the filtered band associated with either multiple reflections between the transmission and reception converters, or reflections between adjacent electrode fingers. Additionally, SAW filters often exhibit an unacceptable sideband rejection and insertion loss, where the presence of the high insertion loss requires peaking the gain of the post-amplifier, resulting in instability and intolerable temperature and aging and shifts of phase. One approach to solving the ripple problem is the utilization of an arrangement with high insertion loss, for example, 22 dB, to minimize the presence of the passband ripple. For most applications, however, insertion loss of 22 dB is unacceptable. One method for providing both low insertion loss and small ripple utilizes group-type electrodes, as discussed in the article "Interdigital Electrode Configurations Cancelling Internal Reflection" by H. Sato et al appearing in Reports of 1975 Spring Meeting, Acoustical Society of Japan, May 1975, at pp. 305-306. An exemplary structure may be composed of a 4-finger-group transducer and a 3-finger-group transducer. In each group, each finger is of .lambda. /4 width with .lambda./4 spacing and opposite polarity from adjacent fingers. This sructure thus provides a cancellation of the internal acoustic reflection between the fingers, which is a major source of ripple. However, the group-type configuration exhibits a low sideband rejection which is often unacceptable.
Apodization is one generally recognized technique for increasing the sideband rejection of a SAW filter. In particular, apodization relates to varying the length of overlap region of adjacent electrodes of opposite polarity, according to a selected function of the electrode positions. U.S. Pat. No. 4,276,524 issued to M. Kadota on June 30, 1981, illustrates an surface acoustic wave device which utilizes apodized electrodes. As described by Kadota, the overlap between adjacent electrodes forms a pattern identical to the impulse response of the surface acoustic wave device. Therefore, by knowing the impulse response desired, the electrodes can be patterned to provide the desired frequency characteristics. The use of apodization with group-type electrodes, however, often introduces unacceptable ripple into the passband.
There exist in the prior art SAW devices which utilize recessed electrodes. One such arrangement is discussed in the article "Surface Acoustic Wave Resonators in Quartz" by W. Tanski, appearing in IEEE Transactions on Sonics and Ultrasonics, Vol. SU-26, No. 2, March 1979 at pp. 93-104. Here, a high-Q SAW resonator is formed which consists of recessed aluminum transducers and shallow reflecting grooves. The Tanski devices, however, are SAW resonators, not SAW transversal filters. Further, the use of recessed electrodes, which reduces reflection between adjacent electrodes, in standard interdigital transducers results in unacceptable passband distortion.
A remaining problem related to SAW filter design is the fact that the requirements of low insertion loss, small passband ripple and good sideband rejection are, in most cases, mutually exclusive. Therefore, there remains a need in the prior art for a SAW filter which simultaneously exhibits all of these qualities.